U.S. Pat. No. 3,332,446 describes a conduit that consists of a metal inner tube for carrying a cryogenic medium, an outer tube arranged concentrically with respect to the inner tube and spaced some distance from it, and a spacer member. The spacer member is a strip-shaped member, which is spirally wound on the inner tube and contacts both the inner tube and the outer tube over the entire length of the piping. One of the side edges of the spacer member abuts the inner tube, and its side edge that faces outwardly with respect to the inner tube is in contact with the outer tube. The spacer member is composed of a heat insulating plastic material, such as polyethylene.
DE 1 936 609 B2 describes a flexible conduit for conveying cryogenic fluids, which consists of an inner tube, a spacer member spirally wound on the inner tube, a cooling line wound on the spacer member in the opposite winding direction, a metal intermediate sleeve arranged above the cooling line and concentrically with respect to the inner tube, a spacer member spirally wound on the intermediate sleeve, and an outer tube. In addition, a layer of thermal insulation that consists of several layers of metallized plastic foil is provided between the intermediate sleeve and the outer tube.
Previously known conduits cannot be used for conveying cryogenic fluids in the vicinity of, for example, nuclear power plants or radioactive materials, since they contain materials that are not stable over extended periods of time within the radius of action of high-energy radiation.